Parasitic wasp larvae clean house by sanitizing cockroaches

Make sure they don't pick up germs while eating the roach from inside out.

The life history of Ampulex compressa, the emerald cockroach wasp, is likely to make you squirm. A female wasp stings a cockroach—momentarily paralyzing its front legs—and drags it by its antennae to her burrow, where she glues a single egg to the roach’s leg. Once the larva hatches, it feeds carefully on the internal organs of the still-living insect, keeping it alive until the larva is ready to pupate. At that point, the larva spins a cocoon inside the roach's exoskeleton and finally emerges as an adult, leaving the carcass of the now-dead cockroach behind.

The cockroach serves as both the restaurant and the meal for the wasp larva, but roaches are a notoriously filthy species that harbor and transmit some pretty nasty pathogens like Salmonella, E. coli, and the eggs of various roundworms. How do the larvae protect themselves from these pathogens while dining on the cockroach’s internal delicacies?

In this week’s issue of PNAS, a group of researchers report that emerald cockroach wasp larvae produce an antimicrobial secretion that helps sanitize the insides of their cockroach hosts.

By peering into holes in cockroaches’ abdomens (which the ever-resourceful scientists covered with coverslips), the researchers observed the wasp larvae depositing small drops of a clear oral secretion into the roaches’ body cavities. They found this secretion had at least two compounds—(R)-(-)-mellein and micromolide—with known antimicrobial properties. (R)-(-)-mellein is known to inhibit the processing of some proteins related to hepatitis C, and micromolide is known to work against the bacteria responsible for tuberculosis.

The scientists extracted these compounds from the larvae’s secretions and tested them against two bacteria commonly harbored by cockroaches: Serratia marcescens, a Gram-negative bacterium, and Staphylococcus hyicus, a Gram-positive one. Together, the two compounds were very effective at inhibiting the growth of each of these pathogens, reducing bacterial growth by more than 50 percent. Further testing showed that (R)-(-)-mellein was more effective at keeping S.marcescens at bay, and micromolide was the better inhibitor of S. hyicus.

(R)-(-)-mellein and micromolide aren’t known to occur together anywhere else in the natural world, which suggests this combination may be an evolutionary adaptation unique to the emerald cockroach wasp. Because of their rather bizarre life history, these wasps likely encounter a huge number of pathogens during their lifetime. Having a broad range of antimicrobial compounds available to them may be extremely advantageous.

Thanks to this new knowledge about their antimicrobial properties, these compounds could contribute to future medicines and drug therapies. The researchers also acknowledge there are likely other unidentified compounds inside the wasps’ secretions that may help us fight disease in the future.

"Gram-negative bacteria are bacteria that do not retain crystal violet dye in the Gram staining protocol. In a Gram stain test, a counterstain (commonly safranin) is added after the crystal violet, coloring all Gram-negative bacteria with a red or pink color. This is because of the existence of an outer membrane preventing the penetration of the stain."

I think what is also interesting is that the implication is that cockroaches survive the salmonella and roundworm eggs but succumb to the wasps. I suppose that cockroaches are so prolific that no-ones's going to notice losing a good chunk of them to salmonella and roundworms. However, they may also have bacteriocidal/bacteriostatic agents that we might be interesting in exploiting. This reminds me of studies on crocodiles, which have been reported to be very resistant to infection despite living in gucky waters (http://www.medscape.com/viewarticle/775470).

Some of those parasitic wasps are just damn crazy amazing. First the tarantula one, now this. The way they evolved blows my mind.

That said. If there is a god, and if he did create all this, I am not convinced I want to meet the being that woke up one day and said....

" Today I shall create a flying insect that will paralysis another insect so that she can lay an egg that will hatch and slowly eat the second insect from the inside out, while it is still alive. Yep. Thats what I am going to do today."

That god, at least in my eyes, has some serious issues that need to be worked out.

Further, considering this is all just a byproduct of evolution. Seriously. WTF happened to create these mini monsters.

These wasps sound like something a twisted scientist who really hated roaches would bio-engineer. [shiver]

Nope, that's not the most gruesome thing a parasitic wasp can do.

The Dinocampus Coccinellae wasp injects an egg into a ladybug and after the larvae has gotten its fill then proceeds to slip out of the still alive ladybug all the while severing the nerves responsible for the ladybug's movement of the legs, and then the wasp larvae makes a cocoon underneath it. Basically the ladybug is a quadraplegic equivalent that starves/dehydrates to death while it serves as a shield for the wasp's pupae.

Kate Shaw Yoshida / Kate is a science writer for Ars Technica. She recently earned a dual Ph.D. in Zoology and Ecology, Evolutionary Biology and Behavior from Michigan State University, studying the social behavior of wild spotted hyenas.